1. Field of the Invention
The present invention relates to a pneumatic tire used for trucks, buses or the like, and in particular, to a pneumatic tire having excellent uneven wear resistance.
2. Description of the Related Art
In conventional pneumatic tires, various measures have been taken against uneven wear, in order to prevent uneven wear of the land portions of the tire and extend the life of the tire. Land portions are portions formed by dividing the ground-contact surface of the tire by circumferential direction grooves or lug grooves.
A conventional pneumatic tire to which a measure against uneven wear has been applied is illustrated in FIG. 2. At a pneumatic tire 100, a center rib 106, a second rib 108, and a shoulder rib 110 are formed from an equatorial plane CL along the tire transverse direction, by circumferential direction grooves 104 formed in the surface of a tread 102.
At the center rib 106 and the second rib 108 of the pneumatic tire 100, a tire radial direction distance (step) H between, on the one hand, a tire transverse direction outer contour line L3 (including the imaginary extension line) along the ground-contact surface, and on the other hand, the ground-contact surface of the shoulder rib 110, is formed so as to be constant.
The shoulder rib 110 has the step H in the direction in which the radius contracts, with respect to the second rib 108 and the center rib 106. Accordingly, when a heavy load is applied to the pneumatic tire 100, the ground contact pressure on the shoulder rib 110 is relatively small, and the ground contact pressure on the center rib 106 and the second rib 108 is equalized. Thus, uneven wear (second rib punch, center rib punch) of the center rib 106 and the second rib 108 can be prevented.
The step H is set to be no more than 0.40 times the tread thickness D at the equatorial plane CL. (The tread thickness D at the equatorial plane CL is the distance, in the radial direction of the tire, from the ground contact surface to the uppermost belt layer 112).
However, because this step H is constant along the tire transverse direction at the shoulder rib 110, there is also a step H at the tire transverse direction outer end side of the shoulder rib. Accordingly, when a usual load to a light load is applied to the pneumatic tire 100, the ground contact pressure becomes even lower at the shoulder rib, at which the ground contact pressure is relatively low as compared to the other portions of the tread when a light load is applied. A drawback arises in that the uneven wear at the outer end side of the shoulder rib is promoted.
In view of the aforementioned, an object of the present invention is to provide a pneumatic tire which exhibits superior uneven wear resistance in all of the land portions formed on the tire.
In order to achieve this object, as a first aspect of the present invention, a pneumatic tire comprising a tread surface having a plurality of grooves extending along a circumferential direction of the pneumatic tire, wherein the grooves define a plurality of land portions the land portions includes; a first land portion which, among the plurality of land portions, is formed the furthest toward a tire transverse direction outer side; and a second land portion formed adjacent to the first land portion in a tire transverse direction, wherein given that a distance, along a tire radial direction, from an imaginary extension line of a tire transverse direction outer contour line along a ground contact surface at the second land portion to a tire transverse direction inner side end portion of the first land portion is H1, that a distance, along a tire radial direction, from an imaginary extension line of a tire transverse direction outer contour line along a ground contact surface at the second land portion to a tire transverse direction outer side end portion of the first land portion is H2, and that a direction toward a tire center is positive, the relationship H1 greater than H2 is established.
In accordance with the first aspect, a step (tire radial direction distance H1) is provided between the first land portion and the second land portion. Thus, the ground contact pressure at the first land portion is lowered relatively, and the ground contact pressures at the first land portion and the second land portion are equalized. Accordingly, even in a case in which a heavy load (a load exceeding 100% when a standard load is considered to be 100%) is applied to the tire and the ground contact pressure is greatest (compared to other portions of the tire) at the tire transverse direction end portion, i.e., the first land portion, uneven wear at the land portion at the equatorial plane side of the second land portion can be reliably prevented.
When a load which is lighter than standard load (100%) is applied (hereinafter, xe2x80x9clight loadxe2x80x9d), in the case of an ordinary pneumatic tire, the smallest ground contact pressure is applied to the tire transverse direction end portion, and thus, there is the concern that the tire transverse direction outer side end portion of the first land portion will wear unevenly. However, in accordance with the present invention, the step H1, which is between an imaginary extension line of the outer contour line of the second land portion and a tire transverse direction inner side (second land portion side) end portion of the first land portion, and the step H2, which is between an imaginary extension line of the outer contour line of the second land portion and a tire transverse direction outer side end portion of the first land portion, are formed so as to be lower toward the outer side end portion side, i.e., H1 greater than H2. Therefore, the decrease in ground contact pressure is suppressed more and more along the direction toward the outer side end portion of the first land portion. Accordingly, as compared to a conventional structure in which the steps are equal (i.e., H1=H2), uneven wear of the outer side end portion of the first land portion can be markedly suppressed.
Namely, by providing the step between the first land portion and the second land portion, uneven wear can be prevented from occurring at the second land portion when a heavy load is applied. Further, by forming the step between the outer contour line and the first land portion so as to decrease from the inner side end portion of the first land portion toward the outer side end portion thereof, uneven wear of the first land portion, which may be great due to the step when a light load is applied, is suppressed.
As a second aspect of the present invention, in the pneumatic tire of the first aspect, the loud portions further include a third land portion formed at a tire transverse direction inner side of the second land portion, and given that a tire radial direction depth of a circumferential direction groove formed between the second land portion and the third land portion is D, the tire radial direction distance H1 falls within the range 0 less than H1 less than 0.3D.
In accordance with the second aspect, when a heavy load is applied, uneven wear of the land portion at the equatorial plane side of the second rib is prevented. When a light load is applied, uneven wear of the tire transverse direction outer end portion of the shoulder rib can be suppressed.
As a third aspect of the present invention, in the pneumatic tire of the second aspect, a first land portion the end portion of the second land portion is chamfered.
In accordance with the third aspect, when there is a step, and in particular, a large step between the first land portion and the second land portion, by chamfering the end portion, the first land portion side end portion of the second land portion can be prevented from protruding out and abutting the road surface. Therefore, uneven wear of this portion can be prevented.